Polyesters, particularly polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate are widely employed for fibers, films and other molded articles due to the excellent mechanical, physical and chemical performances thereof.
As the process for preparing polyethylene terephthalate, for example, there is known a process in which a reaction product comprising an ethylene glycol ester of terephthalic acid and/or a low polymerization degree polymer thereof is prepared by directly esterifying terephthalic acid with ethylene glycol, or transesterifying a lower alkyl ester of terephthalic acid, such as dimethyl terephthalate, with ethylene glycol, or reacting terephthalic acid with ethylene oxide and the reaction product is polycondensed under a reduced pressure in the presence of a polymerization catalyst so that a predetermined polymerization degree is attained. Also polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate are prepared by the same process as described above.
It is well known that the reaction rate and the quality of the resulting polyester are greatly influenced by the type of the catalyst used in the polycondensation reaction described above. As the polycondensation catalyst of polyethylene terephthalate, an antimony compound is most widely used. The antimony compound catalyst has excellent polycondensation catalytic activity and the polyester obtained by using the same has a good color tone.
However, when the antimony compound is used as the polycondensation catalyst, there arises the following problem. That is, if the resulting polyester is continuously melt-spun for a long time, around a spinneret for melt spinning, foreign matter (hereinafter sometimes merely referred to as spinneret foreign matter) is deposited thereby to cause a bending phenomenon of a molten polymer stream extruded through the spinneret, which leads to the occurrence of fuzz and/or breakage of fiber yarns obtained in the spinning step and/or the drawing step.
It is also proposed to use, as the polycondensation catalyst other than the antimony compound, a titanium compound, for example, titanium tetrabutoxide. When using such a titanium compound, the above problem caused by the deposition of the spinneret foreign matter can be solved, while the resulting polyester itself is colored yellow and a new problem, that the thermal stability of the melt decreases, occurs.
To solve the coloration problem, a cobalt compound as a color tone adjustor is generally added to the polyester, thereby to reduce the yellowish tone. Although the color tone (b value) of the polyester is certainly improved by adding the cobalt compound, there arises a problem that the addition of the cobalt compound causes the thermal stability of the polyester melt to decrease and thus the resistance of the polymer to decomposition to decrease.
Japanese Examined Patent Publication (Kokoku) No. 48-2229 discloses that, as another catalyst for preparation of a polyester, titanium hydroxide can be used. Also, Japanese Examined Patent Publication (Kokoku) No. 47-26597 discloses that titanic acid can be used as a catalyst for preparation of a polyester. However, in the former process, titanium hydroxide is hard to form into a powder while, in the latter process, α-titanic acid is hard to store and handle because it is likely to deteriorate. Therefore, the above-mentioned catalyst is not suited for use in the industrial field and also a polymer having a good color tone (b value) is hard to obtain, using the same.
Japanese Examined Patent Publication (Kokoku) No. 59-46258 discloses use of, as a catalyst for preparation of a polyester, a product obtained by reacting a titanium compound with trimellitic acid. Also, Japanese Unexamined Patent Publication (Kokai) No. 58-38722 discloses use of, as a catalyst for preparation of a polyester, a product obtained by reacting a titanium compound with a phosphite ester. Although the thermal stability of the melt of the polyester is certainly improved to some extent by this processes, the resulting polyesters have insufficient color tone. Therefore, further a improvement in the color tone of the polyester is required.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 7-138354 proposes use of a complex of a titanium compound with a phosphorus compound as a catalyst for the preparation of a polyester. Although the thermal stability of the melt of the polyester is certainly improved to some extent by this process, the resulting polyester has an unsatisfactory color tone.